Heterodyne interferometers are utilized in a variety of commercial and noncommercial applications, such as to measure displacement, force, pressure or other physical quantities that create a measurable displacement in a respective transducer. For example, heterodyne interferometers may be utilized in an adaptive optics system to measure movement of a target and/or variations in a medium of beam propagation, such as may be due to atmospheric turbulence. By measuring movement and/or variations in the beam propagation medium, then, the adaptive optics system can correct for such movement and/or variations to accurately image the target.
Heterodyne interferometers include a stable optical source, such as a laser, for providing coherent optical signals. The optical source can provide signals directly to the system or the optical source can be remotely located and connected to the system via optical fibers. Regardless of the location of the optical source, conventional heterodyne interferometers require coherent signals having two different frequencies. In addition, most conventional heterodyne interferometers require that the signals having the first frequency be orthogonally polarized relative to the signals having the second frequency. These heterodyne interferometers are therefore classified as polarizing interferometers. For example, the Hewlett Packard 10715 differential interferometer is a type of polarizing heterodyne interferometer as described in an article by C. Steinmetz, et al., entitled Accuracy Analysis and Improvements to the Hewlett-Packard Laser Interferometer System, SPIE 816, Interferometric Metrology, p. 79 (1987). Similarly, the Hewlett Packard 5527 Laser Position Transducer System is another type of polarizing heterodyne interferometer as described in HP product brochure No. 5964-6190 E entitled Optics and Laser Heads for Laser-Interferometer Positioning Systems (1995).
More specifically, conventional polarizing interferometers include a first laser source for providing a first beam having a first frequency and a first linear polarization and a second laser source having a second frequency and a second linear polarization that is orthogonal to the first polarization state. A polarizing interferometer also includes reference and measurement arms as well as a polarizing beam splitter for separating the first and second beams based upon their polarization such that one of the beams is directed to the measurement arm of the interferometer, while the other beam is directed to the reference arm of the interferometer. Upon returning from the measurement and reference arms, the first and second beams are mixed by a polarization analyzer or other polarization manipulating optical elements so as to create an interference pattern. While the reference arm typically has a fixed or predetermined length, the measurement arm has a length that is defined by the position of a target. As such, as the target is displaced, the optical path length of the measurement arm is accordingly altered. By measuring the phase of the resulting fringes created by the interference of the first and second beams, however, the heterodyne interferometer permits the displacement of the target to be determined.
Nonpolarizing heterodyne interferometers have also been developed. By avoiding the mixing of beams of different polarization states, nonpolarizing interferometers reduce or eliminate the nonlinear errors in the final phase measurement that otherwise arise as a result of polarization crosstalk. See, for example, M. Tanaka et al. “Linear Interpolation of Periodic Error in a Heterodyne Laser Interferometer at Subnanometer Levels,” IEEE Trans. Instrum. Meas., vol. 38, No. 2, pp. 552–54 (April 1989); Jack A. Stone et al., “Wavelength Shift Interferometry: Using a Dither to Improve Accuracy,” Proc. of the Eleventh Annual Meeting of the American Society for Precision Engineering,” pp. 357–62 (Nov. 9–14, 1996); and Chien-ming Wu et al., “Heterodyne Interferometer with Subatomic Periodic Nonlinearity,” Applied Optics, Vol. 38, pp. 4089–94 (1999).
Whereas conventional heterodyne interferometers are adequate in measuring displacement due to movement of a target and/or atmospheric turbulence, such interferometers have drawbacks. Among the drawbacks, conventional heterodyne interferometers are often complex systems that can be expensive to produce. In this regard, conventional phase measuring heterodyne interferometers typically require elements to provide an absolute phase measurement of the resulting fringes created by the interference of the first and second beams, such as by utilizing techniques including fringe counting, zero-crossing phase detecting, and the like. More particularly with respect to adaptive optics applications, conventional lateral shear heterodyne interferometers are typically utilized in wavefront sensors and require ea matrix reconstructor to determine phase, which is required to correct for optical phase differences to the target, which may be represented by the phase measurement of the resulting fringes. Further, many conventional interferometers require an undesirable number of detectors to receive the beam from the measurement arm of the interferometer due to required signal-to-noise ratios.